1. Field of the Invention
The present invention relates generally to a method and apparatus for regenerating caustic etch solutions of the type used in chemically milling aluminum and aluminum alloy parts. More particularly, the invention concerns a simple and straight forward method for regenerating cold filtered sodium hydroxide etching solutions using large reaction vessels which operate at ambient temperature.
2. Discussion of the Invention
Chemical milling is widely used in the prior art for precisely contouring the surfaces of various types of parts which have a particular configuration that makes surface contouring difficult or impossible using conventional milling machines. The technique is particularly useful in milling large sheet metal workpieces to uniform wall thicknesses.
Typically the chemical milling etchant used in large scale commercial aluminum chemical milling operations comprises a sodium hydroxide solution containing a small amount of dissolved aluminum. During the chemical milling process, the sodium hydroxide is consumed and the soluble compound, sodium aluminate, is formed. As the milling process progresses, the etching rate declines in proportion to the gradual decrease in the concentration of free sodium hydroxide and the simultaneous increase in the concentration of the dissolved aluminum. When the dissolved aluminum concentration reaches a certain level, it adversely affects the quality and effectiveness of the etchant and the spent solution must either be pumped out and replaced with fresh etchant, or the solution must be regenerated.
A novel and useful method and apparatus for accomplishing production scale chemical milling is described in U.S. Pat. No. 4,523,973 issued to Nelson. Similarly, a generally typical prior art method for etching aluminum and aluminum alloy bodies is described in U.S. Pat. No. 2,975,041 issued to Holman.
In accordance with prior art practices, when regeneration of the etchant solution became necessary, the regeneration process was accomplished by seeding the spent etchant with alumina trihydrate crystals under closely controlled temperature regimens. During the regeneration process, the dissolved sodium aluminate within the solution reacts with the alumina trihydrate crystals to form more alumina trihydrate and to regenerate sodium hydroxide as follows:
Sodium Aluminate.fwdarw.Alumina Trihydrate+Sodium Hydroxide
Typically, the conventional prior art caustic regenerators used to regenerate the spent etchant typically operated at very closely controlled elevated temperatures of on the order of 130 degrees Fahrenheit. Because the conventional belief of those skilled in the art was that precise temperature control of the crystallization process was imperative, the regenerators often embodied complex and costly temperature control systems. Additionally, the more successful prior art regeneration systems were operated on a continuous flow basis thereby further adding to the cost and complexity of the regenerator systems.
By way of example, one prior art method and apparatus for regenerating an etch solution at an elevated temperature is disclosed in U.S. Pat. No. 4,372,805 issued to Takahashi, et al. The method and apparatus described in this patent includes the steps of withdrawing a portion of the etchant from a reservoir, adding a polymeric flocculent in a thickener to form a solid sulfide and then removing the solid sulfide from the thickener by means of a nozzle. The supernatant liquid in the thickener is then introduced into a crystallizer and water is added. Next, the crystallized aluminum oxide is separated and removed from the crystallizer by means of a centrifugal separator. The alkaline liquid which is discharged from the crystallizer as an overflow supernatant is then introduced into a recovery tank and concentrated by means of an evaporator. Finally the concentrated liquid is removed from the evaporator and returned to the etchant tank with or without the addition of sodium hydroxide and/or sodium sulfide.
U.S. Pat. No. 5,091,046 issued to Hunter et al discloses a process for etching aluminum in caustic solution capable of providing a consistently uniform matte finish like that of the never dump process, but with little waste like the regeneration process. Etching is performed in a solution containing free sodium hydroxide and dissolved aluminum in a ratio between about 0.6 and 2.1 g/l and also containing an etch equalizing agent at a temperature above about 70.degree. C. In column 4, starting at line 63, Hunter describes the "conventional regeneration process" as being "typically operated at a temperature between about 55.degree. C. and 60.degree. C., with an aluminum concentration between about 25 and 30 g/l, and a free sodium hydroxide between about 50 and 70 g/l for aluminum. Hunter continues pointing out that in his invention "the etch temperature is higher than about 70.degree. C., preferably about 80.degree. C."
As will be better appreciated from the discussion which follows, the method and apparatus of the present invention greatly simplifies the process and apparatus for regeneration of the etchant solutions and, for the first time, provides a simple, regeneration system which operates at ambient temperature and requires no complex equipment, instrumentation or controls. Since the method uses large vessels which operate at ambient temperature, no sophisticated and costly temperature control instrumentation is required.